1. Field of the Invention
The invention in general relates to detection systems and in particular to detection systems having a plurality of detector/sending units for reporting the existence of a condition to a central receiving unit.
2. Description of the Prior Art
Detection systems which include a plurality of remote sending units which transmit coded signals to a central receiving unit which decodes the signals to produce an alarm or other indication of a condition at the remote location are well known. The conditions may be the existence of a fire, an intrusion, an emergency or other condition desired to be monitored. Or the condition may be the status of the sending unit, such as the condition of its battery or other sensor status. Systems in which such conditions are reported at periodic intervals are generally known as supervised systems. Because the sending units act independently, two or more transmissions will occasionally overlap, a situation referred to as collision or clash. When a clash occurs, information from the clashing transmissions is lost at the receiving unit. If clash occurs in a supervised transmission, the sending unit appears to be missing or not functioning for that supervisory cycle. The sending unit is then erroneously reported as missing or not functioning. If the two clashing transmitters have identical or very close reporting cycles, their transmissions may become synchronized, resulting in multiple successive clashes.
Prior art systems have attempted to solve the problem of clash by requiring the transmissions from an individual sending unit to be missing for a time equal to several supervisory cycles and by having loose tolerances on the transmitter electronics. The loose tolerances decreases the probability that two or more transmitters in a system will have supervisory cycles that are close enough to cause multiple successive clashes. However, this approach is effective only when the duration of the transmissions are very short relative to the supervisory period. Further, a detection system must operate continuously for years, and in a large system with, say, thirty or more transmitters installed over a wide area with varying ambient conditions (which can change the cycle periods) the probability is unacceptably high that two or more transmitters will at some time have reporting cycles that are sufficiently close to cause synchronized clashing.